1. Field of the Invention
The present invention relates to a cellulose acylate film, a process for producing a cellulose acylate film, and a polarizing plate and a display device using the subject cellulose acylate film.
2. Description of the Related Art
In recent years, following an increase in the size of a liquid crystal display, the liquid crystal display is started to be used in new utilities such as a television set, and a demand for an image quality improvement becomes remarkably high. Above all, contrast and viewing angle dependency of color are a problem inherent to a liquid crystal display device and are a performance which is especially high in necessity for improvements.
For this reason, in addition to conventional TN modes, there have been proposed new liquid crystal modes such as IPS, VA and OCB modes. However, even in all of these liquid crystal modes, it is difficult to obtain sufficient contrast and viewing angle dependency by a liquid crystal cell alone. Thus, it is general to compensate bireflingence which the liquid cell has by some retardation film, thereby designing to improve the above-described characteristics.
Hitherto, a cellulose acylate film was used as a protective film for polarizing plate. In recent years, there is proposed a method of positively imparting retardation to this cellulose acylate film and using it as an optical compensating film. For example, JP-A-2003-170492 discloses a method for using a tenter-stretched cellulose acylate film as a retardation film of a VA type liquid crystal mode.
However, an existing cellulose acylate film suffers from a problem to be solved. Namely, its haze would be elevated with an increase in retardation and, as a result, the contrast of a liquid crystal display device is lowered.
As a countermeasure thereto, JP-A-2003-301049 discloses a method in which in a stretched cellulose acylate film having secondary particles having an average particle size of from 0.1 μm to 1.5 μm and containing primary fine particles having an average particle size of 0.001 μm or more but less than 0.1 μm, the content of additives in the neighborhood of the secondary particles is made larger than the average content of the additives in the film, thereby suppressing aggregation of the fine particles and reducing haze. However, this method is still insufficient in lowering haze and, therefore, should be further improved. However, in the cellulose acylate film, since its haze increases following stretching, there are involved problems that a lowering of the contrast is remarkable. Thus, improvements were required.